This invention relates to a carriage body for public transportation vehicles and more particularly concerns a light alloy roof structure for such carriage bodies.
A light alloy body structure for coaches, buses, trolley buses, streetcars and the like is disclosed in the U.S. Pat. No. 4,059,303 of the same Applicant. In essence, that structure is formed of a latticework skeleton of longerons and uprights forming the two body sides. The pattern of such structure is repeated in the vaults and roof, whereat the uprights are replaced by vault bows or arches and crossbeams.
In the cited patent, the latticework skeleton structures of the sides and roof, being much alike, are separated by longerons which frame and support the individual latticework skeletons. The evident rationality of such structure design concept has been confirmed also in vaulted shed roof, where it is now commonly employed. However, it has been found that if in an advanced schematization process the carriage body shell is likened to a tubular girder subjected to various stress conditions, then the load bearing and twist resisting functions, i.e. the stiffening of the shell against deformation as an articulated parallelogram, are mainly performed by the sides, while the roof or vaults having the primarily static function of supporting their own weight, plus any added weight, such as that of a snow layer or other occasional loads. Thus, the advisability of adopting a latticework structure for the roof becomes debatable, and the more so because that structure is an expensive one which involves, moreover, the lining thereof with sheet metal throughout.